$Pre

$Pre

is a global variable whose value, if set, is applied to every input expression.

Details

  • Unless $Pre is assigned to be a function which holds its arguments unevaluated, input expressions will be evaluated before $Pre is applied, so the effect of $Pre will be the same as $Post.
  • $Pre is applied to expressions, while $PreRead is applied to strings which have not yet been parsed into expressions.

Examples

Basic Examples  (2)

By default, $Pre is not defined, meaning the input is evaluated directly:

Save input expressions in held form:

Evaluation works as expected:

Restore $Pre to its default:

The variable inputs holds the inputs in unevaluated form:

Wolfram Research (1988), $Pre, Wolfram Language function, https://reference.wolfram.com/language/ref/$Pre.html.

Text

Wolfram Research (1988), $Pre, Wolfram Language function, https://reference.wolfram.com/language/ref/$Pre.html.

BibTeX

@misc{reference.wolfram_2020_$pre, author="Wolfram Research", title="{$Pre}", year="1988", howpublished="\url{https://reference.wolfram.com/language/ref/$Pre.html}", note=[Accessed: 25-February-2021 ]}

BibLaTeX

@online{reference.wolfram_2020_$pre, organization={Wolfram Research}, title={$Pre}, year={1988}, url={https://reference.wolfram.com/language/ref/$Pre.html}, note=[Accessed: 25-February-2021 ]}

CMS

Wolfram Language. 1988. "$Pre." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/$Pre.html.

APA

Wolfram Language. (1988). $Pre. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/$Pre.html